add cracking affine cipher tutorial

x4nth055 · x4nth055 · commit 30ab1fe2b1b6 · 2023-12-08T16:10:36.000+01:00
diff --git a/README.md b/README.md
@@ -52,6 +52,7 @@ This is a repository of all the tutorials of [The Python Code](https://www.thepy
     - [How to Lock PDFs in Python](https://thepythoncode.com/article/lock-pdfs-in-python). [(code)](ethical-hacking/pdf-locker)
     - [How to Create a Custom Wordlist in Python](https://thepythoncode.com/article/make-a-wordlist-generator-in-python). ([code](ethical-hacking/bruteforce-wordlist-generator))
     - [How to Implement the Affine Cipher in Python](https://thepythoncode.com/article/how-to-implement-affine-cipher-in-python). ([code](ethical-hacking/implement-affine-cipher))
+    - [How to Crack the Affine Cipher in Python](https://thepythoncode.com/article/how-to-crack-the-affine-cipher-in-python). ([code](ethical-hacking/crack-affine-cipher))
 
 - ### [Machine Learning](https://www.thepythoncode.com/topic/machine-learning)
     - ### [Natural Language Processing](https://www.thepythoncode.com/topic/nlp)
diff --git a/ethical-hacking/crack-affine-cipher/README.md b/ethical-hacking/crack-affine-cipher/README.md
@@ -0,0 +1 @@
+# [How to Crack the Affine Cipher in Python](https://thepythoncode.com/article/how-to-crack-the-affine-cipher-in-python)
diff --git a/ethical-hacking/crack-affine-cipher/affine_cipher_decrypt.py b/ethical-hacking/crack-affine-cipher/affine_cipher_decrypt.py
@@ -0,0 +1,92 @@
+# Import the needed libraries.
+import string
+from colorama import Fore, init
+
+# Initialise colorama.
+init()
+
+
+# Function to get Euclidean Algorithm.
+def extended_gcd(a, b):
+    """
+    Extended Euclidean Algorithm to find the greatest common divisor
+    and coefficients x, y such that ax + by = gcd(a, b).
+    """
+    if a == 0:
+        return (b, 0, 1)
+    else:
+        g, x, y = extended_gcd(b % a, a)
+        return (g, y - (b // a) * x, x)
+
+
+# Function to get the modular Inverse
+def modular_inverse(a, m):
+    """
+    Compute the modular multiplicative inverse of a modulo m.
+    Raises an exception if the modular inverse does not exist.
+    """
+    g, x, y = extended_gcd(a, m)
+    if g != 1:
+        raise Exception('Modular inverse does not exist')
+    else:
+        return x % m
+
+
+# Function to decrypt our message.
+def affine_decrypt(ciphertext, a, b):
+    """
+    Decrypt a message encrypted with the Affine Cipher using
+    the given key components a and b.
+    """
+    alphabet = string.ascii_uppercase
+    m = len(alphabet)
+    plaintext = ''
+
+    # Compute the modular multiplicative inverse of a.
+    a_inv = modular_inverse(a, m)
+
+    # Iterate through each character in the ciphertext.
+    for char in ciphertext:
+        # Check if the character is in the alphabet
+        if char in alphabet:
+            # If it's an alphabet letter, decrypt it.
+            # Find the index of the character in the alphabet.
+            c = alphabet.index(char)
+            # Apply the decryption formula: a_inv * (c - b) mod m.
+            p = (a_inv * (c - b)) % m
+            # Append the decrypted character to the plaintext.
+            plaintext += alphabet[p]
+        else:
+            # If the character is not in the alphabet, keep it unchanged.
+            plaintext += char
+
+    # Return the decrypted plaintext.
+    return plaintext
+
+
+# Function to peform brute force attack.
+def affine_brute_force(ciphertext):
+    """
+    Brute-force attack to find possible keys for an Affine Cipher
+    and print potential decryptions for manual inspection.
+    """
+    alphabet = string.ascii_uppercase
+    m = len(alphabet)
+
+    # Iterate through possible values for a.
+    for a in range(1, m):
+        # Ensure a and m are coprime.
+        if extended_gcd(a, m)[0] == 1:
+            # Iterate through possible values for b.
+            for b in range(0, m):
+                # Decrypt using the current key.
+                decrypted_text = affine_decrypt(ciphertext, a, b)
+
+                # Print potential decryption for manual inspection.
+                print(f"Key (a={a}, b={b}): {decrypted_text}")
+
+
+ciphertext = input(f"{Fore.GREEN}[?] Enter Message to decrypt: ")
+
+# Perform a brute-force attack to find potential decrypted message.
+affine_brute_force(ciphertext)
diff --git a/ethical-hacking/crack-affine-cipher/requirements.txt b/ethical-hacking/crack-affine-cipher/requirements.txt
@@ -0,0 +1 @@
+colorama

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	`1`	`+# [How to Crack the Affine Cipher in Python](https://thepythoncode.com/article/how-to-crack-the-affine-cipher-in-python)`